Multilayer printed circuit boards have conventionally been produced by steps including preparing a stack in which a plurality of one-sided printed circuit boards or double-sided printed circuit boards having a circuit formed by etching, are laminated by pressing via an adhesive layer such as a glass woven fabric prepreg, and a hole is opened in the stack using a drill or laser, after which a conductive layer is formed on the hole walls by plating or the like for electrical connection between the layers.
In recent years, methods of directly forming wiring patterns by printing methods are being investigated as substitutes for the conventional methods of producing multilayer printed circuit boards using etching or plating. For example, a method of forming a wiring pattern by ink jet printing (Patent document 1) and a method of producing a multilayer printed circuit board by forming a conductive layer and a hole-formed insulating layer on a substrate by a printing method (Patent document 2) have been proposed.
With these production methods, it is possible to produce a multilayer printed circuit board without using large-scale equipment such as pressing and plating equipment. An additional advantage is very high material usage efficiency, because the conductor ink or insulator ink can be printed only on the necessary sections.
At the same time, advances are being made in thickness reduction and high densification of circuit boards, to meet the needs for miniaturization and lighter weights of electronic devices in recent years. In addition, for electronic devices in the fields of data transmission and information processing, it is becoming ever more essential to accomplish efficient securing of mounting areas for mounting of high-performance parts. In the attempt to secure mounting areas, research has been carried out on miniaturization of surface mounting parts, narrow-pitch formation of terminals and fine patterning of boards, SMT (surface mounting technology) for high-density mounting of parts on board surfaces, and Advanced SMT which is a higher level of the same technology.
However, the numbers of active element parts (chip parts) have been increasing to meet the needs of higher functioning for electronic devices. As the number of passive element parts (capacitors, inductors and registers) that perform electrical regulating is likewise increasing, the mounting areas of such passive element parts often occupy more than half of the entirety. This has constituted an obstacle against miniaturization and higher performance of electronic devices.
Techniques for building passive element functions into boards are also being investigated. Such techniques promise not only miniaturization, but also effects such as improved reliability, by eliminating the conventional solder joints used for electrical connection between surface mounting parts and circuit boards, increased freedom of circuit design, improved electrical characteristics by reducing parasitic capacitance since passive elements can be effectively positioned internally, thereby shortening wiring lengths, and lower cost by eliminating the need for surface mounting.
Passive element-forming materials have therefore been developed with the purpose of providing passive element functions inside boards. For example, materials plated with relatively high-resistance metals have been used as resistors for built-in passive elements (Patent documents 3 and 4). There has also been proposed a method of forming resistors by ink jet printing, without the process of plating or etching (Non-patent document 1).
The present inventors have also proposed an ink comprising carbon black particles dispersed in a thermosetting resin, as a material that allows resistors of desired sizes to be formed at desired locations using an ink-jet apparatus (Patent document 5).